This invention relates to rigid, closed-celled polyisocyanate-based foams, particularly polyurethane, polyurethane-isocyanurate and polyurethane-urea foams prepared using halocarbon blowing agents.
It is well-known to prepare such polyurethane foams by reacting an organic polyisocyanate (including diisocyanates) with an active hydrogen-containing compound in the presence of a blowing agent or agents. Generally speaking, such blowing agents are volatile organic compounds which are liquids at room temperature. The polymerization reaction between the active hydrogen-containing compound and the polyisocyanate giving a polyurethane is exothermic as is the trimerization reaction of isocyanate giving an isocyanurate polymer. The reaction exotherm is sufficient to vaporize the blowing agent, which then becomes encapsulated in the liquid phase of the polymerizing reaction mixture resulting in the formation of cells. The formation of cells causes the reaction mixture to expand and form a foam which subsequently cures to become a rigid closed-celled polyurethane foam.
A problem frequently encountered is that of preventing an unacceptable degree of shrinkage of partially cured foam during the aging or curing period following manufacture. A further problem often encountered in cured foams, especially in applications where polyurethane foam may be exposed to subzero temperatures, below 0.degree. C., for extended periods of time, is shrinkage or dimensional stability.
Such shrinkage or poor dimensional stability is frequently observed either where blowing agents used to impart the cellular structure to the foam have a high diffusion rate through the urethane polymer such as, for example, carbon dioxide. Or where the blowing agents employed have atmospheric boiling points such that when foams containing these blowing agents are subjected for prolonged periods to low temperatures, the blowing agent condenses.
During the aging or curing period following the manufacture of the foam, the blowing agent employed depending on its permeability and diffusibility through the polymer may gradually diffuse out of the cells. Loss of blowing agent from the cells can eventually lead to shrinkage and poor dimensional stability of the foam. The condensation of blowing agent leads to greatly reduced internal cell pressures and eventually resulting in shrinkage and/or poor dimensional stability of the foam. A further disadvantage of having condensed blowing agent present can be a potentially harmful effect for any polymer or other plastic material present. The polystyrene inner liner used in the production of some refrigeration units can be attacked by the condensed blowing agent.
The shrinkage and poor dimensional stability of some polyurethane foams used for cold-insulation purposes may be controlled to an extent by for example increasing the foam density.
It would therefore be desirable to provide a process for the manufacture of polyurethane foams which have reduced susceptibility to shrinkage and improved dimensional stability. At the same time, it would be an additional advantage if such a process could provide for a reduction or elimination of the commercial dependency on especially "hard" chlorofluorocarbon (CFC) blowing agents. The "hard" CFC blowing agents are those compounds in which all hydrogens of the carbon backbone have been replaced by a halogen, normally fluorine and chlorine, in contrast to "soft" CFCs which have at least one hydrogen atom remaining on the carbon backbone. Such "hard" CFCs are suspected of destroying the earth's protective ozone layer by migrating up through the troposphere to the stratosphere and participating indirectly or directly in the chemical reactions of ozone.